A vehicle climate control system performs two primary functions: temperature regulation and dehumidification. In a system having a variable capacity refrigerant compressor, this is generally achieved by adjusting the stroke of the compressor based on the cooling load, and then reheating the cooled and dehumidified air as required to regulate the temperature of the passenger compartment air at the desired temperature. In this type of control the principal feedback variable is passenger compartment temperature, and the relative humidity within the passenger compartment varies significantly depending on cooling load, ambient humidity, and so on.
While widely used, the above-described approach exhibits significant drawbacks in terms of both compressor power consumption and passenger comfort. System efficiency suffers due to overcooling and then reheating for purposes of temperature regulation, while passenger comfort suffers under conditions where the relative humidity of the passenger compartment falls below a comfortable level. Clearly, reducing the compressor capacity under certain conditions will improve both vehicle fuel economy and passenger comfort without degrading the passenger compartment temperature control.
For the above reasons, it has been proposed to measure the relative humidity in the passenger compartment and to regulate the system operation to control both temperature and humidity. However, an accurate humidity sensor significantly increases system cost, and therefore limits the system applicability to primarily luxury vehicles. Accordingly what is needed is a cost effective approach for controlling both temperature and humidity in a vehicle climate control system.